The present invention relates to an antenna unit having an antenna device mounted on a substrate made of a conductive material and particularly to an antenna unit, which has less distortion in its directivity due to secondary radio wave signals radiated from the substrate.
In recent years, progress has been made in downsizing antenna units. This makes possible to install a global positioning system (GPS) antenna unit for a GPS navigation system in a dashboard of a vehicle.
When the GPS antenna unit is installed in a vehicle, it receives radio wave signals from satellites through a front windshield. The radio wave signals are reflected between the substrate of the GPS antenna unit and the front windshield. As a result, levels of the received signals vary depending on the position of the GPS antenna unit. To solve this problem, a device having a radio absorbing material on the top surface side of the substrate is proposed as disclosed in JP-A-11-330847.
However, when the GPS antenna unit is installed in a dashboard, directivity distortion occurs in some cases even though the radio absorbing material is installed. In such cases, the radio wave signals from the GPS satellites cannot be received. This results from many dielectrics and metal parts installed inside the dashboard. The radio wave signals are reflected off metal parts installed on the rear surface side of the antenna unit. As a result, the radio wave signals are radiated or diffracted from the rear surface side of the substrate to the top surface side, creating directivity distortion.
In a small antenna unit for a transmitting device such as an electronic toll collection (ETC) system, directivity distortion may occur as well. In such a unit, a substrate used as a ground is reduced in size and hence not sufficient for grounding. Therefore, the radiated radio wave signals are diffracted to the rear surface side of the substrate. The diffracted radio wave signals are reflected off surrounding parts and radiated as secondary radio wave signals from the surrounding parts, resulting in directivity distortion.
Even the method disclosed in JP-A-11-330847 is applied and a radio absorbing device is installed around the antenna device on the top surface side, this problem cannot be resolved.
The present invention therefore has an objective to provide an antenna unit that reduces distortion in directivity of the antenna caused by secondary radio wave signal radiation or diffraction.
An antenna unit of the present invention has a radio absorbing device on the rear surface side of a substrate. With this configuration, radiation of radio wave signals from the rear surface side can be reduced even when dielectrics or metal parts are installed around the antenna device. As a result, directivity distortion of the antenna due to interference with radiated radio wave signals from the dielectrics or metal parts can be effectively reduced.
The radio absorbing device is mounted in an area that tends to create an intense electrical field. Secondary radio wave signals tend to be radiated or diffracted from such an area including a corner or an edge of the substrate. Mounting the radio absorbing device only in the area can reduce directivity distortion at low cost.
A radio absorbing material can be used for the radio absorbing device. When the radio absorbing material is used, it can be provided on an entire rear surface of the substrate. This ensures reduction of secondary radio wave signal radiation from the substrate.